Arduino Programming – Part 5:
User-defined functions
ME 120
Mechanical and Materials Engineering
Portland State University
http://web.cecs.pdx.edu/~me120
User-Defined Function
• What is it?  A user-defined function is a
function that you write yourself. Opposite to
Built-in functions such as digitalWrite, delay,
etc..
• Why?  to simplify and organize your code.
• How?  Built your function outside of your
void loop and void setup. Call it from
anywhere. It will be treated like a built-in
function.
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Example: Countdown
• You would like to built a countdown from 10
to 0 that you could see on your serial
monitor.
• Without creating any function, the code is:
void setup() {
Serial.begin(9600);
int i;
for (i=10; i>=0; i--){
Serial.println(i);
delay(1000);
}
}
//Initialize serial port communication
//goes from 10 to 0, decrement of 1
//print the value of i on serial monitor
//wait 1 sec between each value
void loop() {
}
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Example: Countdown
• Let’s built a function that we call “countdown”
void setup() {
Serial.begin(9600);
countdown();
}
//Initialize serial port communication
// call the function countdown
void loop() {
}
void countdown(){
int i;
for (i=10; i>=0; i--){
Serial.println(i);
delay(1000);
}
}
// create the function countdown
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Example: Countdown
• Let’s add a parameter: the value of the
countdown start
void setup() {
Serial.begin(9600);
countdown(15);
}
//Initialize serial port communication
// call the function countdown
void loop() {
}
void countdown(int start){
int i;
for (i=start; i>=0; i--){
Serial.println(i);
delay(1000);
}
}
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Example: Countdown
• Let’s add a second parameter: the value of
the time delay between the counts
void setup() {
Serial.begin(9600);
countdown(15, 2000);
}
//Initialize serial port communication
// call the function countdown
void loop() {
}
void countdown(int start, int deltaT){
int i;
for (i=start; i>=0; i--){
Serial.println(i);
delay(deltaT);
}
}
6
Example: Countdown
• We want to know the sum of all the numbers
counted in the countdown.
void setup() {
Serial.begin(9600);
int result;
result = countdown(15, 2000);
}
void loop() {
}
int countdown(int start, int deltaT){
int i;
int sum=0;
for (i=start; i>=0; i--){
Serial.println(i);
delay(deltaT);
sum = sum+i;
}
return (sum);
}
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Summary of user-defined functions
• You chose the name
❖
Make sure the name is not already used
• You chose the type of return value
• You choose the number and type of inputs
❖
•
Input types are declared in the function definition
int countdown(int start, int deltaT){…}
❖
Input variables are used in the body of the function
• Variables in the function are local
❖
Calling function is not affected by local variables and logic in
the function
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